AbstractDiscusses the nature and extent of base oil‐additive and additive‐additive interactions. The first interaction is in the bulk oil phase, where solution chemistry is involved. The second interaction occurs at the wear surfaces. Various experimental techniques (DSC, TFOUT, FRT, Four ball) were used to measure interactions. In an examination of base oil‐additive‐interactions on oxidation stability, various species of base oils exert a strong influence on the performance of antioxidants. In additive‐additive interactions, the data suggest a strong concentration of dependent interaction patterns among the various additives and the base oils. Mechanistic studies are recommended to elucidate some of the observed phenonema.